update default release branch name to main. Add --releaseBranch parameter to allow overriding this.

[enovales]

Proposed Changes

The default branch name for new GitHub repositories is now main. This diff changes the default release branch name for the scaffold to main, to align with that, but also adds a --releaseBranch configuration switch, to allow overriding that during initial scaffold setup (for example, if you're bootstrapping the scaffold into an existing repository).

Types of changes

What types of changes does your code introduce to MiniScaffold?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

Build dependencies were changed to target F# 5.0, so that string interpolation can be used. This isn't critical, so it can be backed out if necessary.

[TheAngryByrd]

Thanks!